Development of Technologies for Active Wavefront Control of Advanced Gravitational Wave Detectors

Abstract

The era of gravitational-wave astronomy started with the detection of a binary black hole coalescence on the 14th of September 2015 by the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO). By the end of 2017, a total number of 11 gravitational wave events have been detected by LIGO and Virgo detectors. One of these events, GW17081, produced by the coalescence of a binary neutron star signaled the dawn of multi-messenger gravitational astronomy, revealing invaluable information about the physics occurring in such cataclysmic event. The work presented in this thesis is part of the ongoing global effort to improve the sensitivity of current detectors and thus improve both the detection rates and the information that can be gleaned from each detection. The sensitivity of terrestrial interferometric detectors are broadly limited by coating thermal noise and quantum noise. Increasing the circulating laser power and injecting vacuum-squeezed light are employed to reduce the quantum noise. However, the ability to implement these measures and their efficacy is fundamentally limited by absorption-induced wavefront distortion within the interferometer. At the time of writing this thesis, aLIGO detectors are struggling to increase the input power above approximately 30 W and the current observed level of squeezing at aLIGO Livingston and Hanford Observatories are 3dB and 2.2 dB respectively, partly due to wavefront mismatch. New technologies are urgently required to diagnose these issues. In this thesis, I will will describe the development of a new technologies for the solution of this problem: an advanced “phase camera” that can examine individual RF sideband fields used to control and sense the interferometer and new adaptive optics for active wavefront control and mode-matching within the interferometer. The new phase camera measures the complex amplitude of a coherent field that is frequency-offset from a reference field, and records the transverse profile with high spatial and temporal resolution. Furthermore, it does so without the use of scanning mirrors and thus is suitable for use during both detector commissioning and low-noise operation. This thesis also describes the development of new thermally-actuated mirrors for adaptive wavefront control and mode matching in aLIGO. The two designs presented are the thermal-bimorph mirror and the compression-fit mirror. Both of which show a large and linear actuation range, and low higher-order aberrations. They are currently scheduled for deployment to assist with mode matching between the squeezed light source and the signal recycling cavity of aLIGO and can be extended to other optical interfaces during the detectors A+ upgrade.